As is well known, flat panel displays (FPDs) have become mainstream as image display devices in recent years, the FPDs being typified by a liquid crystal display, a plasma display, a field emission display, an OLED display, and the like. Progress is being made toward reducing the weight of those FPDs, and hence glass substrates used for the FPDs are also currently becoming thinner (being formed as a glass film).
Further, there is a growing use of an organic light-emitting diode as a plane light source, such as a light source for interior illumination, which emits only monochrome (for example, white) light, unlike a display that uses TFTs to blink light of three fine primary colors. Further, when an illumination device that uses the organic light-emitting diode includes a glass substrate having flexibility, a light-emitting surface is freely deformable. Therefore, from the viewpoint of ensuring sufficient flexibility, there is also promoted further thinning of the glass substrate to be used for the illumination device.
In addition, as disclosed in Patent Literature 1, a thin glass sheet such as a glass film is utilized for reducing weight of window sheet glass by laminating the glass sheet on both surfaces of a transparent plate made of an organic resin.
The glass film to be utilized in various fields as described above is cut to a predetermined size depending on a product size or the like. As a cutting method therefor, there is employed a method of cleaving the glass film by locally heating and cooling the glass film, and propagating an initial crack formed in the glass film by a thermal stress generated through the heating and cooling (see, for example, Patent Literature 2).
Specifically, in such a technique, under a state in which the glass sheet is placed on, for example, a processing table made of metal, the glass film is locally heated by applying a laser beam along the preset cleaving line of the glass film, and then the heated region resulting from the heating is locally cooled by a cooling device. Accordingly, the thermal stress is generated in the glass film to propagate, along the preset cleaving line by the thermal stress, the initial crack formed in advance at a leading end portion of the preset cleaving line so that the glass film is cleaved.
However, in the above-mentioned technique, the processing table made of metal or the like is held in contact with a back surface of the glass film, and hence the heat applied to the glass sheet through the localized heating using a laser or the like is easily dissipated toward the processing table having a high thermal conductivity. Thus, it is difficult for the thermal stress to act on the glass film efficiently. In particular, such a problem becomes more conspicuous as the glass film becomes thinner.
To address such a problem, for example, as disclosed in Patent Literature 3, it is conceivable that the thermal stress is caused to act along the preset cleaving line of the glass sheet under a state in which the back surface of the glass sheet is not brought into contact with the processing table or the like by blowing a gas on the glass sheet from below to float the glass sheet.